1. Field of the Invention
The present invention generally relates to heat exchangers for use in regulating the temperature of a patient's blood during surgery, and more particularly to a micro-conduit heat exchanger with enhanced sealing between the heat transfer fluid and the patient's blood.
2. Description of the Related Art
"Heart-lung" machines are known in the medical field. One component of these machines is a blood oxygenator. Blood oxygenators are typically disposable and serve to infuse oxygen into a patient's blood during medical procedures such as heart surgery. Most commercially available blood oxygenators employ a membrane-type oxygenator, which comprises thousands of tiny hollow fibers having microscopic pores. Inside the membrane oxygenator blood flows around the outside surfaces of these fibers while a controlled oxygen-rich gas mixture is pumped through the fibers. Due to the relatively high concentration of carbon dioxide in the blood arriving from the patient, carbon dioxide from the blood diffuses through the fibers' microscopic pores and into the gas mixture. Due to the relatively low concentration of oxygen in the blood arriving from the patient, oxygen from the gas mixture diffuses into the blood through the fibers' microscopic pores.
Most blood oxygenators also employ a heat exchanger to precisely regulate the temperature of a patient's blood. The heat exchanger usually includes one or more relatively large conduits housed in a vessel. The patient's blood is continuously pumped through the conduits, while a heat transfer fluid such as water flows through the vessel around the conduits, or vice versa. The heat exchange medium is either heated or cooled to maintain the patient's blood at a desired temperature.
One example of a commercially successful blood oxygenator is sold under the designation MAXIMA.TM. by Medtronic Corp. In the MAXIMA blood oxygenator, the heat transfer fluid (water) flows inside relatively large diameter metal tubes while blood flows on the outside of the tubes within the vessel. The TERUMO brand oxygenator uses a different configuration, where blood flows inside relatively large diameter metal tubes. In the BARD WILLIAM HARVEY HF-5700 blood oxygenator, the blood flows outside plastic tubes that contain a flow of temperature-regulated water.
Heat exchangers in blood oxygenators are subject to a number of design constraints. The heat exchangers should be compact due to physical space limitations in the operating room environment. Also, small size is important in minimizing the internal priming volume of the blood oxygenator due to the high cost and limited supply of blood. However, the heat exchanger must be large enough to provide an adequate volumetric flow rate to allow proper temperature control and oxygenation. On the other hand, blood flow rate or flow resistance inside the blood oxygenator must not be excessive since the cells and platelets in the human blood are delicate and can be traumatized if subjected to excessive shear forces resulting from turbulent flow.
One way to meet the above requirements is to provide a heat exchanger with improved heat exchange efficiency. A more efficient heat exchanger can provide adequate temperature control in a compact space with minimal priming volume. Some improvement in heat exchange efficiency may be achieved by the use of certain heat transfer fluids. However, because of toxicity considerations, blood oxygenator heat exchangers generally utilize water as a heat transfer fluid.
Another way to increase the heat exchange efficiency is to increase the surface area of contact between the blood and the heat transfer fluid. While this can be done by simply enlarging the heat exchanger, the above considerations severely limit the size of the heat exchanger to relatively small proportions. Another approach would be to increase the number of heat exchanger conduits while decreasing their size. This would result in increased surface area contact with the blood while maintaining a small volume.
However, a number of problems can be encountered with smaller heat exchanger conduits. For example, finding an appropriate material for the heat exchanger tubes is difficult. Many materials are not suitable for this application due to the extremely low tolerance of contamination and toxicity. While metals have been successfully used in past blood heat exchangers, metals present a number of difficulties. First, since metal tubes of small diameter must be manufactured more precisely, they are more expensive than larger tubes. Furthermore, this expense is compounded due to the increased number of heat exchanger tubes required in such a design.
Plastic materials, while less expensive than metals to manufacture in small sizes, pose different problems when used for heat exchanger conduits. Plastics have poor heat transfer characteristics, and therefore, their use necessitates an even greater surface area to efficiently achieve a desired rate of heat exchange. Moreover, many desirable plastic materials can pose problems due to their relatively low critical surface tension. As a result, plastic materials do not "wet" easily which also makes it difficult to "prime" the interior of the conduits before injecting blood into them to avoid air bubbles in the conduits. This also causes difficulties in reliably bonding to these materials. Bonding to blood heat exchanger conduits is critical to ensure that there is absolute isolation of the heat transfer fluid from the blood. Furthermore, the bonding material must be non-toxic and biocompatible since it will likely be in contact with the blood.